Over the lifetime of an IC, various mechanisms result in the degradation of transistors, other components and interconnects of an IC. Hot-carrier injection (HCI) occurs when charge carriers (electrons or holes), propelled by excess kinetic energy, stray into a nonconductive region of a transistor, such as the gate dielectric of a metal-oxide semiconductor field-effect transistor (MOSFET). Bias temperature instability (BTI) results from applying the same voltage to the control terminal of a transistor over time. Subsequent toggling of the transistor has to overcome the resulting voltage bias, slowing its switching speed. Charge traps, which are pockets of conduction in a dielectric layer, can be formed over time and eventually cause the dielectric to break down and form a short circuit in a transistor. Electromigration occurs when voltage surges cause electrons in the interconnects to drift into the transistors and remain there.
Degradation caused by HCI, BTI, charge-trap formation and electromigration generally reduces the intrinsic speed of a transistor and the circuit in which the transistor is employed. In early stages of degradation, the circuit speed may be restored by increasing the voltage at which the circuit is driven. Later on, after the drive voltage has been raised as far as possible, the frequency at which the circuit is driven speeds should be reduced to accommodate the ever-decreasing circuit speed. Eventually, however, the circuit will begin to behave erratically and will eventually cease to function at all.
Degradation tends to become more predominant as new technologies have allowed the sizes of transistors, other components and interconnects to become ever smaller. Various techniques have been devised to detect and compensate for IC degradation. Those techniques have been more or less successful at increasing IC performance, extending IC life or predicting impending IC failure.